1. Field of Invention
The present invention relates to an organic light-emitting display (OLED). More particularly, the present invention relates to an organic light-emitting display having a power line divided into multiple sets such that a voltage terminal is coupled to the center of each power line set and that the voltage terminals from various sets are all coupled to a power supply through a medium fabricated using a low resistant material.
2. Description of Related Art
One of the earliest recordings of dynamic images can be found in documentary movies. Later on, when the cathode ray tube (CRT) was invented, television is brought into each family. With the advent of the computer age, CRT is adopted as a display monitor for the desktop computer. Despite its popularity, the radiation gun design always poses some hazard to human health and increases the bulk of each unit. Thus, less bulky and less hazardous alternatives to the CRT are sought after.
Flat panel displays are the results of intensive research to reduce size and weight of each display. Many types of displays belong to this category. They include liquid crystal display (LCD), field emission display (FED), organic light-emitting display (OLED) and plasma display panel (PDP). Organic light-emitting display is a type of self-illuminating display also commonly referred to as organic electroluminescence display (OELD). Major characteristics of an OLED are: dc low voltage driven, high luminance, high efficiency, high contrast value and light. In addition, an OLED is capable of producing a spectrum of color including the three primary colors red (R), green (G), blue (B) as well as white. Hence, OLED has the greatest potential to become the dominant type in the next generation of flat panel displays. Aside from being thin, light, energy-saving, self-illuminating and having a high resolution, a device fabricated using the OLED technique also has a wide viewing angle, a brilliant color contrast and a relatively low cost of production. With these advantages, it has been broadly applied as a LCD or backlight in an indicator panel, a mobile phone, a digital camera and a personal digital assistant (PDA).
According to the driving method, OLED can be classified into passive matrix driven type and active matrix driven type. A passive matrix driven OLED has a rather simple structure and does not require any thin film transistor (TFT) to drive the circuit and hence has a lower production cost. However, the passive matrix OLED has only moderate resolution and poor displaying capacity. Furthermore, as size of the display panel is increased, energy consumption is increased and working life is shortened. On the other hand, active matrix OLED technique—can be applied to form a large display screen with a wider viewing angle, a higher luminance level and a quicker response. The only drawback is that it has a higher production cost than a passive matrix OLED.
According to the driving method, flat panel displays can be divided into voltage driving and current driving types. The voltage driving type is normally applied to the TFT-LCD. Different voltages are applied to the data lines to attain different gray levels so that a full panel of colors is produced. Voltage driven TFT-LCD is technically mature, operationally stable and non-expensive to implement. The current driving type is normally applied to an OLED. Different currents, instead of voltages, are applied to the data lines to attain different gray levels so that a full panel of color is produced.
In an active OLED, a large current will flow in power lines around the pixel array. In general, these power lines are fabricated from thin sheets of metal so that its resistance is usually large and hence may lead to a rather large voltage drop after the passage of a current across a short distance along the power line. However, the actual voltage applied to a pixel will affect the size of a current passing into the OLED and ultimately the luminance level of the pixel. Consequently, voltage differential across a power line often leads to a non-uniformity of pixel brightness level.
FIG. 1 is the simulated voltage drop along a power line in a conventional organic light-emitting display. As shown in FIG. 1, a power line 102 is coupled to a voltage terminal having a voltage Vdd. Assume a current I flows along the power line 102 due to the voltage Vdd and the power line 102 is dissected into several sections with each section of the power line having a resistance R. As the current I flows along the power line 102, the voltage at a first secondary power line 104 is Vdd, the voltage at a second secondary power line 106 is Vdd−IR, the voltage at a third secondary power line 108 is Vdd−2IR and so on. In general, the nth (where n is a positive integer) secondary power line 110 will receive a voltage Vdd nIR. When the voltage drop in the secondary power line is large, the voltage across the organic light-emitting diode in each pixel between the positive and the negative terminal will drop correspondingly. In other words, there is a great reduction in the current passing through the organic light-emitting diode leading to a considerable drop in the overall brightness level. Consequently, there will be considerable difference in luminance between the light-emitting diodes powered by the first secondary power line and the last secondary power line.
In addition, a plurality of voltage terminals can be used in the display panel to shorten the distance between the connected power lines and reduce the number of secondary power lines (that is, reduce the sum of current flowing through the power line). With this set up, voltage drop across each secondary power line will be reduced. FIG. 2 is the simulated voltage drop along another power line design in a conventional organic light-emitting display. In FIG. 2, the power lines are divided into four sets, a first power line set 202, a second power line set 204, a third power line set 206 and a fourth power line set 208. Each power line set is coupled to M (M is a positive integer) secondary power lines and N (N is a positive integer) secondary power lines. The terminal for each power line set is closer to the Nth secondary line (that is, farther away from the Mth secondary line). Assume the current flowing through each secondary power line in the group of M secondary power lines is I1 and the current flowing through each secondary power line in the group of N secondary power lines is I2. The voltage at the Mth secondary voltage will be Vdd−(M*I1*R) and the voltage at the Nth secondary voltage will be Vdd−(N*I2*R). Using the first power line set 202 and the second power line set 204 as an example, the voltage between the Nth secondary power line in the first set 202 and the Mth secondary power line in the second set 204 is considerable. Thus, with this setup, considerable non-uniformity in brightness level still exists between neighboring pixels in the panel.